Make a second horizontal crease one third from the top of the letter where the bottom of the letter had been folded to.
Tuck the bottom into this crease and fold the top over it. The letter will be folded into thirds. It will fit any standard envelope.
If you are folding the letter so the address faces out the envelope window, fold the letter toward the back instead of the front. The letter address will appear through the envelope window, but it will still be folded in thirds.